texts: Examples of absorption lines for main sequence stars

texts:
Where might I find a detailed table or figure containing the absorption lines and their corresponding wavelengths for each type of main sequence star?

I'm trying to get the chemical abundance of some elements on a G0V spectrum but I don't know which lines I'm looking at.


texts:
The SWAN C/2020 F8 comet is passing near the Earth, but from Wikipedia

The comet has dimmed a small amount since May 3.
It was expected to possibly reach 3rd magnitude in May, but is now expected to hover closer to magnitude 5.

What is the cause of this dimming?


texts: How to calculate the mass of a galaxy? I have its apparent angular diameter and its distance. A H2 cloud at the galaxy's disc has a velocity of 410 km/s. How to calculate the mass of the galaxy?


texts: What is the Galactic Latitude and Galactic Longitude? How is different from the latitude and longitude we use on Earth?

texts:
It has to something with the positioning of sun and galactic center of the milky way but I wasn't able to understand how for a given star, its co-ordinates are calculated.

Some explanation would be appreciated.


texts: Why is the Earth-Moon distance not the same at each perigee/apogee?

texts: I wonder why the Earth-Moon distance is not the same at each perigee/apogee. Isn't the Moon's orbit a fixed ellipse with Earth at one of the foci? If it is so, shouldn't the distance at perigee/apogee be a fixed value?


texts: How long until the stars are no longer visible by the human eye due to light pollution?

texts: I was curious to how long it will be until the stars are not able to be seen due to light pollution. I started wondering this after reading Yummie's Flash comic Knite.


texts: What is the unit used for star brightness due to skyglow?

texts: What is the name and unit of the quantity in which star brightness due to skyglow is measured? How can it be measured from RAW image files of stars?


texts: What direct or indirect observations of dust can one make by eye or with binoculars?

texts:
The solar system and space in general has plenty of dust, and dust has major impacts on observing both visually and in radio astronomy.

If I wanted to "see dust" in space, or see its effects, using my eyes or at most a pair of regular binoculars, where could I look and what would I look at in order to say to myself "yep, that's dust!" or a direct result of dust's properties in space?

NOTE: It doesn't necessarily have to be interstellar dust!


texts: Why does disorder (entropy) increase in the same direction of time as that in which the universe expands?

texts:
Why does disorder  (entropy) increase in the same direction of time. as that in which the universe expands?

Why can't entropy decrease in the same direction in which it increases ?


texts: What decides the direction in which the accretion disk spins?

texts:
Planets lie on the same plane because of the accretion disk formed during the Protostar stage, as I read in this question.

I also read about the collision of particles in the gas cloud causing the overall spin to be in just one direction. But what decides the direction in which the accretion disk spins relative to the direction of the core?

(I'm thinking that it might be because of some primary conditions - maybe the direction in which the core is spinning.)


texts: Why do astronomers not use past tense when discussing observations of the universe

texts:
Describing a galaxy 70 million light years away, I consider it incorrect to state for example "it has an active black-hole at its center". It did 70 million years ago, who knows what is out there now. I could understand using the present tense for objects within the solar system and even nearby (less than say 100 ly) but not for such distant objects.

Is there some convention to use present-tense for all observations, or am I being a pedant?


texts: Can we see individual stars in other galaxies?

texts: Just wondering if we can examine individual stars from other galaxies, or if we are pretty much stuck with the billions that are in the Milky Way?


texts: Can we see traces of exo-moon formation?

texts: Our Moon was likely formed by the collision of a Mars-size object with the Earth soon after the planets first formed. Would traces of such an event be detectable by observing the protoplanetary disc around a young star?


texts: is there any mathematical formula, paper or anything that shows the probability of a meteor hitting a particular spot of moon's surface?

texts: According to this article http://news.nationalgeographic.com/2016/02/160209-meteorite-death-india-probability-odds/ a science professor at Tulane University made a paper to calculate the probability of a person being hit by a meteor on Earth . And also an astronomer made a calculation (not mentioning the procedure) with a similar figure. My question is, since the moon has no atmosphere and more meteors reach the surface, is there any mathematical formula, paper, etc. to calculate how likely a spot in moon's surface it's likely to be hit by a meteor?


texts: How did Kepler determine the orbital period of Mars?

texts:
As I understand it, Kepler used the orbital period of Mars, along with observational data of Mars' and the sun's position in the sky to derive the orbits of Earth and Mars.  (As described, here: https://faculty.uca.edu/njaustin/PHYS1401/Laboratory/kepler.pdf)

But how did he determine the orbital period of Mars without already knowing the orbit of the Earth?


texts:
How does the Sun burn without oxygen?

It might not be burning but a big part of the society speaks of it as burning.

So how does it work?


texts: What is the temperature about 10 or 15 feet below Mars' surface?

texts: I was wondering this for a school paper I'm writing.


texts: Difference between J2000, FK5 and ICRS coordinate systems? Which one does the Yale Bright Star Catalog use?

texts:
I have no background in astronomy. I have been wanting to write code to make star charts for a given location and time, which led to this question.

I figured out that I would need to convert the coordinates of stars from the equatorial coordinate system to a horizontal coordinate system defined by a time and place of interest.

I decided to use the Yale Bright Star Catalog for this purpose.

From what I could gather, the Yale Bright Star Catalog does not explicitly mention the coordinate system it uses, it mentions the epoch to be J2000 frequently though and reports RA, DEC. So, it is my understanding that the coordinate system should be an equatorial one with J2000 as the equinox.

As a novice, I could understand the horizontal, equatorial and ecliptic coordinate systems. However, when I started using an astronomy library to write the code, I realised that in practice, there are equatorial coordinate systems such as FK5 and ICRS with minute differences; and that ICRS is the coordinate system adopted by the IAU.

So, which coordinate system is the YBSC catalog in, exactly?

I have noticed that mentioning the epoch, but not the exact coordinate system is the somewhat confusing, but standard way to report coordinates. Example: coordinates in M33's Wikipedia Page, or CDS name resolver.

So, in general, which equatorial coordinate system should be considered 'default' when just the epoch is mentioned as in the examples above? Should it be FK5, ICRS, or something else?


texts: Does this galaxy resemble the Milky Way, and is it relatable as the "template" for the Milky Way?

texts:
Taking a look at this photo of comet 2I/Borisov and a background galaxy (2MASX J10500165-0152029):

And compare it to this rendition of the Milky Way:

The two galaxies slightly resemble each other. Is there a relationship between these two? Was this galaxy used in defining the Milky Way's shape?


texts: What safety concerns should an amateur astronomer take?

texts:
I'm an amateur trying to take a shot at recreational stargazing. I've read about how looking a some eclipses without the appropriate instruments (i.e, with naked eyes) is bad for the eyes. To what extent is this true? Is staring at the phenomena bad or is even a quick glance dangerous?
Are there any more phenomena like eclipses I have to watch out for to protect myself?


texts: Approximate conversion of redshift 'z' to a time and/or distance, when reading papers?

texts:
I'm looking for some form of "rough and ready" formula to convert between redshift z value, years since BB, and distance, so that when I read an astronomy paper and it discusses an event that occurred at z=10+/-0.5, or a quasar at z=7, I can get a rough idea when the event occurred and how far away they are saying the quasar is, as context.

I'm aware that this is an inherently vague question, since the times and distances depend on the model and convention chosen, and also that I'm not specifying proper distances/comoving coordinates, etc.

My assumption is that most current models and their parameters,  that are mainstream accepted and used, will be based on very similar parameters of very similar models (if not the same model), such as latest Planck parameters, or similar. They might differ more due to assumptions about very early times, but redshift is inherently >~380 k years, and if they did vary a lot for times beyond that timeframe, we'd have real issues.   In other words, I suspect/hope that any differences in answers due to model variation, won't significantly change the answer to this question. So the only question will (hopefully) be, which convention or type of metric/distance will be appropriate.

If an assumption needs making (such as the meaning of 'distance' to be applied, or the zero point used for time: BB or end of inflation etc) please make an assumption  most likely to help me, and I'll edit the question to clarify if needed, once I see what is too vague in the question as initially worded.

Thank you !


texts: Can planets have orbits other than elliptical or circular orbits? All of the planets in our solar system have either a circular orbit or an elliptical orbit (perhaps like Pluto).


texts: Why haven't Earth and Venus got any tiny moons? Or have they?

texts:
Why haven't some meteoroids gotten caught in Earth's or Venus' orbit?

AFAIK most meteors are tiny fragments from comets. Shouldn't some comet tail sometime have passed Earth orbit at velocities suitable for our planet to capture such fragements? And 100 000s of asteroids have been detected. Why haven't the inner planets gotten asteroid like moons like the outer planets have? The asteroid belt isn't too far away.

Earth's large Luna might clean them away, but that won't explain Venus' lack of tiny moons.


texts: Do we know the availability of any material suitable for solar cells on the moon?

texts:
Disclaimer: I have no formal training in any scientific field.

Have we located any material suitable for solar panels already on the moon? Or does it appear that they would have be imported?


texts: Spectra of electromagnetic radiation emitted during solar flare

texts: During solar flare, electromagnetic energy is released at a wide range of wavelengths but the most dominating are x-rays and extreme ultraviolet rays. Why is that intensity of these wavelengths is greatest?


texts: Circular velocity in axisymmetrici potential

texts:
If a matter distribution is spherically symmetric, the circular velocity is straightforwardly related to the mass via:

$v_{circ} = \sqrt{GM/r}$

In an axisymmetric potential, on the other hand hand, we have

$v_{circ}(R) = \sqrt{R\frac{d\Phi}{dR}}\neq \sqrt{GM/r}$

with $\Phi$ the gravitational potential.

What I don't quite understand is the following: the potential of the Milky Way is not spherical. How is the measured velocity then reliably related to a mass?


texts: Why didn't the parachutes on the Apollo 14 Command Module bump into each other?

texts: It seems like they would need some mechanism to keep them apart, but in this photograph I don't see anything telling, unless it's those small pilot chutes that are keeping them apart?


texts: Star rotational axes and Solar system orientations

texts: How does the orientation of stars axes of rotation and any planetary systems relate to the plane of the galaxy? Is there any relationship that has been detected, are they grouped from their parental nebulae. Do galactic magnetic fields have coherent orientations that would affect nebula growth and consequent stellar growth.


texts: Spotted fast, VERY bright, moving object in the sky tonight in Gex, France

texts:
Tonight around midnight, I spotted a very bright moving object in the sky. Confused and intrigued with my sighting, I decided to go on google to find some answers. It clearly was not a plane as it was far to bright and did not flash. It was not a shooting star as it was not as fast and moved for around 1 minute in the sky before disappearing. I firmly believe it was not a satellite as it was, again, really too bright (and I had spotted 3 other satellites prior to this, it really did not compare). After searching around on various forums describing similar sightings, many people suggested it could have b ISS. However, after checking it’s location today (July 24-25 (23:59-00:00) it was over Australia, meaning I would not have been able to see it.
I even checked websites regarding possibilities that it may have been Jupiter, Saturn or even Mars as they a notorious for being particularly bright during the July-August period. However, pardon my lack of astronomy knowledge, I assume they would not be moving at such a high speeds. I also checked a star map showing their locations, at this moment in time they are facing south and the object I spotted was facing east.
This leads me to my final point. The only evidence I found online which resembles my sighting was similar to this mans sighting in his video: (https://www.dailymail.co.uk/news/article-3410791/What-bright-object-flying-sky-Man-claims-filmed-UFO-Florida.html)
However, when I spotted “this” object, it was not at all green but a bright, white light. He also refers to his sighting as a ‘UFO’ and I personally don’t believe in aliens hovering around earth in little spaceships.
Does anyone have any idea what it could have been? I am very curious about this now.
Thank you!
*edit: to the kind stranger who suggested the other forum post as a possible solution (Star like light moving in the sky, what could it be? [closed]) what he describes is not the same, the object was as bright as the brightest star in the sky that night, and travelled over the whole stretch of the sky that I could see, it came over the Jura mountain range and vanished towards the Alps


texts: Can you see man-made lights on the dark side of the Earth from the surface of the moon with the naked eye?

texts:
This question is prompted from a comment on this answer:

Could a person on the surface of the moon see man-made lights on the dark side of the Earth with the naked eye? If not, how much magnification would be required to do see them?

The closest evidence I could find from a quick search was this picture from Salt Lake City Schools' Website:
The dark part of Earth is a bit hard to see here, though it can be seen more easily in the full resolution image. You can certainly see something toward the bottom edge in the dark area, but I can't tell for sure if it is lights from Southeast Asia/Oceania or something else.

Edit: As pointed out in the comments, the image of Earth in this photo wasn't actually taken from the moon, but from NASA's Terra and Aqua satellites and combined with the lunar imagery in a 2002 visualization created by NASA to commemorate the 30th anniversary of Apollo 17.


texts: Period coincidence of Planet Nine with furthest Kuiper belt objects

texts:
A lead scientist of the Planet Nine theory said that "The period with which it goes around the sun is a rational multiple of the periods of all the furthest Kuiper Belt objects"

To what degree of precision can Konstantin Batygin make that kind of measurement? Has it been backed up by other physicists with a degree of certainty?

He also says that they think it's near it's aphelion "because it hasn't been seen, so it's far away", not because of the above period coincidence, why can't they say where it is more convincingly?

Does that mean that they have very strong mathematical evidence for Planet Nine?


texts: Dozens of Oblong Green Objects in Google Sky

texts:
In reviewing Google Sky today for the first time, zoomed in at the southern end of the Aquarius constellation, there's dozens of oblong objects, many of which are green. Considering their general uniformity I'm assuming that they are moving objects, meaning the exposure time stretched them out and caused color distortion showing them in green.

Has anyone come across these before and found an explanation as to what they're supposed to be?

Extreme example is 22h17m54.44s - 2°49'58.88" as of May 5, 2020.
Another more typical example is  22h14m58.38s - 3°10'58.58".

If you can't see anything green at either of those locations, just zip around. They are all over the place in this area.


texts: Why do type Ia supernovas produce more iron than type II

texts:
My course book on astronomy states the following.

Older stars seem have higher oxygen abundances than iron. Explanation is that back in the days when these older stars were being formed type II supernova's were common, while type Ia weren't. So later, when the type Ia was becoming more common, the - now younger - stars were formed with higher iron abundances.

Why are type Ia supernova's better for iron enrichment than type II, and were these type II supernova's in any way better for higher oxygen abundances - or just less good at producing iron (and why)?


texts: Calculating Air Mass at Zenith for observing sites above sea level?

texts:
Air mass relates to how much of the Earth's atmosphere the light from an observed object must pass through before being observed.

At Zenith, and sea level, the value of "air mass" is set to 1.

Light from objects lower to the horizon at an observing site must travel through more atmosphere, and hence the brightness of the object is somewhat diminished. In this situation the value of air mass increases above 1.000, presenting greater dimming for the observed object from increased atmospheric light scattering.

As example, when making precise observations of the magnitude of an object in astronomy, there is a need to calculate the air mass for an object at the time of observation, determined in large part by its altitude angle from zenith, or from the geometric horizon.

There are various formulas for calculating air mass. Precise calculations consider several factors and can be quite complicated.

For more details see - Air Mass (Astronomy). Wikipedia.

My question:  Is there a simple formula to calculate the air mass at Zenith (only), for observing sites about sea level.

Such a calculation provides a comparison of the merit of an observing site at higher elevation over one at lower elevation in regard to the factor of air mass.

I imagine for most elevations the factor is quite small. Naturally there are plenty of other factors to consider when it comes the the quality of an observing site - humidity, frequency of clear skies, human light sources and light pollution, etc.

To my understanding, the atmosphere in most calculations is considered to be 9 km in thickness.

A very simple calculation could be the site elevation subtracted from 9 km, divided by 9 km, providing a ratio value somewhat less than 1. However this calculation does not account for the changing density of the atmosphere with increasing elevation.

If there is a simple formula, what is it?


texts: In multiple articles I have seen the age of a star, within the milky way, referred to as its redshift (typically denoted by $z$). I know that $z$ can be calculated as $z=\frac{\lambda_{obsv} - \lambda_{emit}}{\lambda_{emit}}$ but what I can't find is how this is then used to calculate the age of a star within our galaxy? What characteristic does the redshift show us (spin, velocity, temperature, etc.) and how is this used mathematically to calculate age? Or is it just a method to determine where it falls on the HR diagram?


texts: What will future populations of stars be like?

texts:
As stars slowly using up hydrogen and ejecting heavier elements into space, the future stars will form from nebulae that are more metal and helium rich.

The question is: how does the increased ratio of helium and metals affect star formation and stellar evolution?

Does more helium and metals in the core at the beginning means the star begins it's life in an advanced state so it dies faster?


texts: Can a lunar eclipse completely 'turn off' the mooon, i.e. like a New Moon? Or is it always just a 'shading' of the moon (sometimes red in color)?


texts: Why was the Vera C. Rubin Observatory LSST Camera tested with a broccoli and a pinhole?

texts:
The SLAC National Accelerator Laboratory video Vera C. Rubin Observatory LSST Camera (caution! annoyingly loud music, lower volume before playing) shows a Romanesco broccoli (which I'd assumed was a 3D printed mathematical surface of that name until I checked and found out that this is the name of an actual vegetable i.e. Brassica oleracea) as the object of a pinhole camera projecting its image on to the astoundingly large CCD array.
There are likely several advantages to using the pinhole rather than a well-corrected projection lens, is it possible to say what they are?
Is it also possible to elaborate on the choice of object for the camera?
See also:

slac.stanford.edu Sensors of world’s largest digital camera snap first 3,200-megapixel images at SLAC
BBC: Vera Rubin: Super telescope's giant camera spies broccoli
What is the LSST now? Where does LSST end and Vera C. Rubin Observatory begin?

Caution! annoyingly loud music, lower volume before playing:
https://youtu.be/qc_iscV1uA0


texts: Which kind of properties can we get for cosmic ray particles hitting on an optical ccd?

texts:
It is very common that we meet cosmic ray particles in optical images recorded by CCDs.
You can see the "snowflakes" in the hubble images below:

Generally we should remove them in order to get photometry or spectroscopic info.

However, there may be useful info we can derive, for example, can we get their directions or energies statistically before they arrive at a CCD?

There is some correlation between the ADU value of a cosmic ray point and its real energy?


texts: Is a three body gravitating system doomed to collapse?

texts:
Suppose we have two gravitating bodies, which are rotating around each other. They are bodies and are affected by deformation caused by tidal forces. Moving tidal waves suck energy from the axial rotation of these bodies. After some time their axial rotation will be synced with orbital rotation and tidal waves will be stopped. Like Moon orbital rotation around Earth is synced with its axial rotation. Maybe I missed some additional effects but it seems to be a quite accurate assumption.
Except for some special cases, if we have three orbiting bodies, their orbital rotation cannot be synced with axial rotation altogether.  Tidal waves in such a system will exist and suck energy indefinitely. And the only source of energy is potential body energy. Will they fall?
I understand that such effects should be very small; it hardly can be measured in the solar system, because the variation of mass of planets and the Sun causes a much higher effect.
But anyway are there any measures that support the existence of such an effect for something? Or is there theoretical proof that this effect does not work for some reasons I missed?


texts: When was visible light for the first time created?

texts:
"A few minutes into the expansion, when the temperature was about a billion (one thousand million) kelvin and the density was about that of air, neutrons combined with protons to form the universe's deuterium and helium nuclei in a process called Big Bang nucleosynthesis. Most protons remained uncombined as hydrogen nuclei. As the universe cooled, the rest mass energy density of matter came to gravitationally dominate that of the photon radiation.

After about 379,000 years the electrons and nuclei combined into atoms (mostly hydrogen); hence the radiation decoupled from matter and continued through space largely unimpeded. This relic radiation is known as the cosmic microwave background radiation."

The question now is, when you count back starting with the cosmic background radiation which is now in far red (2,7K) at what time was there for the first time visible light, or was is right after the creation of atoms about 379,000 after the big bang?


texts: If space was n>3 dimensional, would clouds of particles still turn into 2D disks due to gravity?

texts:
I don't fully understand why clouds of particles which turn into galaxies and solar systems usually result in an (approximately) 2D shape, but whatever. Would they still result in an almost 2D shape if our space was n dimensional with $ n > 3 $ and the laws of physics were the same (adapted for that $ n $)? Or would it be an (approximately) $ n-1 $ dimensional shape?

It would be of interest to have an answer to this question under the assumption that the attractive force was still proportional to $1/r^2$ even when the dimensionality is greater than 3.


texts: Is it odd that our Sun has so many planets?

texts: The internet is abuzz with the new evidence of the possible ninth planet in our Solar System. With those nine objects, the asteroid belt, the Kuiper belt, and everything else in our Solar System - the number of objects which orbit our star is pretty massive. Is this odd for a star its size? It's known that our Sun is a comparatively small star, so does having so many planets make it an oddity? Or do larger stars tend to have dozens or planets, even hundreds?


texts: (Thought experiment) if we put a huge mirror into space, could we see back in time to the big bang?

texts:
This is an addition to this question which is closed:

By putting a mirror in space, would we be able to see into the past?

Imagine there is a supernova a hundred years ago. Lots of people see it but nobody has a decent telescope. A hundred years later we point a powerful telescope in the opposite direction, and find a huge alien mirror a hundred light years away, that allows us to watch the supernova.

My question is, what is the limit of such a technique?

Could we see right back to the big bang if the mirror was large enough and far enough away?

Please note this is not a question about whether the telescope or mirror are realistically possible.
Assume we are using some superior alien technology.


texts: Does the relative motion between planets other than Venus and Earth form shapes such as the Flower of Venus?

texts:
I was reading about the Pentagram/Flower of Venus, and how it is caused by a 13:8 ratio between their orbits. I found this GIF particularly fascinating.

My question is, if the ratio changed, would they form other kind of "fractal" shapes? Does this happen between other planets? What would be the condition for the ratio so that the relative motion forms a symmetrical shape like the Cardioid in the case of Venus and Earth?


texts: Why are some universal entities round and others are flat?

texts:
I've been wondering for a while now why some objects in the universe (such as stars, planets, etc.) are round, while others are flat (galaxies, planetary rings and protoplanetary disks).

I know these shapes come to be because of gravity. In planets, the gravity from the core pulls everything to the centre, thus, rounding the planet (same with stars). But with Saturn, for example, why have its rings formed as rings and not as a spherical layer of ice surrounding the planet?

The same with the Kuiper belt and asteroid belt, I would presume the mechanics there are the same as with a planetary orbit, hence, why they are belts and not clouds. But then what about the oort cloud? Why is that a huge cloud in the outermost solar system and not a huge ring of ice.

As for the solar system itself, I've read that the solar system is kind of egg-shaped, but shouldn't it be just like the galaxy (or the other way round)?

I've tried looking into this and wondering about what different laws of physics may apply in each case, but it's all really confusing.


texts: Is C/2012 S1 (aka ISON) the fastest comet on record to-date?

texts:
This source may be nebulous (no pun intended!) for some. It writes to say

  Starting at around 95,000mph at the start of this month, Comet ISON will eventually reach an astonishing 845,000mph on November 28th when it catapults round the Sun at perihelion

I'm no astronomer; the only comets I even know the names of are

Shoemaker Levy
Swift Tuttle
Wild
Halley Bopp

To the best of my knowledge (feel free to poke me in the rib if i'm wrong!) none of the above list achieved a 3-digit velocity at perihelion. In contrast, going by the figure quoted above, S1 is expected to achieve a mind-boggling 375 km/s!

Is C/2012 S1 the fastest comet on record to-date?


texts: Why is sodium such a common ion for in ion tails?

texts: I was doing research about ion tails of planetary bodies and noticed that ion tails composed of sodium were common. For example, Mercury and the Moon both have ion tails made of sodium. Why is sodium, as opposed to other ions, commonly seen in ion tails?


texts: Will there be any more planets come into our solar system and make it their "home"?

texts: Could we have more planets come into our system and get trapped by our suns gravitational field and rotate?


texts: Should we expect clues from a gamma ray burst causing massive extinctions on Earth?

texts:
I remember reading an article on the subject highlighting the periodicity of this event throughout history supposing it could be caused by the rotation of the sun around the galaxy.

My question is: Do we have any clues that can confirm this hypothesis? What should we expect to see? Shouldn't it leave any remains?


texts:
This question came to my mind after reading this.

I think it is safe to assume that the supposedly thrown camera would not reach the event horizon in one piece. So my question is :

If I dropped a TV camera into a black hole, will it be destroyed before it reaches the event horizon, and if so, why?


texts: If the Sun disappears right now, when the absence of its gravity affects on Earth?

texts:
Imagine if the Sun disappears right now. When the Earth will be affected by its absence? Does it affect immediately, or it takes almost 8 minutes? (The time that light travels from the Sun to the Earth.)
So if I ask this question in another way, the changes in Space-Time will affect immediately or it will travel by light speed?


texts:
The Wikipedia page for the blink comparator fails to mention who invented it. Many other pages extol the importance of the device, but we cannot find any mention of an inventor.

Wikipedia does mention that Ben Mayer invented the The Projection Blink Comparator (PROBLICOM), which is a low-cost improvement on the blink comparator. However, he did not invent the original device.


texts: Voyager 1 and 2 detected a 20-fold increase in plasma density, significantly different refractive index?

texts:
As reported by Nature.com, Voyager 1 and 2 detected a 20-fold increase in plasma density going from the heliosphere out to interstellar space. Does that 20-fold difference in density imply a difference in refractive index, and if so, how much?

Here's the abstract of the (paywalled) paper:

  The heliopause is the boundary between the hot heliospheric (solar wind) plasma and the relatively cold interstellar plasma. Pressure balance considerations show that there should be a large (factor of 20 to 50) density increase across the heliopause. Here we report electron density measurements from the Voyager 1 and 2 plasma wave instruments near and beyond the heliopause. The plasma density in the outer heliosphere is typically about 0.002 cm⁻³. The first electron density measured by the Voyager 2 plasma wave instrument in the interstellar medium, 0.039 cm⁻³ ± 15%, was on 30 January 2019 at a heliocentric radial distance of 119.7 au. The density jump, about a factor of 20, confirms that Voyager 2 crossed the heliopause. The new density is very similar to the first density measured in the interstellar medium by the Voyager 1 plasma wave instrument, 0.055 cm⁻³, on 23 October 2013 at a radial distance of 122.6 au. These small differences in the densities and radial distances are probably due to the relative locations of the spacecraft in the boundary layer that forms in the interstellar plasma just beyond the heliopause.


texts: Comet Neowise c2020 f3 will return in ~6800 years, how can we be sure Earth will be in the right place to see it then?

texts:
I have read in several places that the orbital period of comet Neowise c2020 F3 is about 6800 years. In several popular media sources it is said that we will be able to see the same comet after ~6800 years.
How is it known that the Earth will be at exactly the right place in order for it to be visible?


texts: Can some stars not emit any energy in the visible spectrum?

texts: Stars convert mass into energy. Even converting the slightest mass into energy is immense because the speed of light is so great and $E = mc^2$. This means that stars have to emit large amounts of energy. So can they emit so much energy in a form of electromagnetic radiation that is not visible light? Resulting in a star that cannot be detected using our senses(+ a telescope). If so what problems could this star pose?


texts: Is the Bondi Radius the Same as the Schwarzschild radius?

texts:
$$2MG/c^2$$

This is the expression for both, but some authors use one name and it has confused me. Is there any distinction?


texts: How are gravitational lensing and dark matter related?

texts: In a lecture which I attended at my institution on gravitational lensing, the lecturer linked gravitational lensing and dark matter research. Can anyone explain what is the relation of both?


texts: Does there exist a code that implements Henyey method for interior parts of the star?


texts: How to calculate moon overhead and underfoot time?

texts:
For hunting purposes, some hunters follow the moon phases to hunt accordingly. To give some context, here's some links:

Lunar cycle's effects on hunting: https://www.acurite.com/blog/weather-stations-with-moon-phase-for-hunting-fishing
Solunar theory: https://en.wikipedia.org/wiki/Solunar_theory

I would like to know how to calculate the time when the moon is directly overhead in the sky (moon overhead) and when the moon is directly underfoot in the opposite side of the earth's sky (moon underfoot). Those are the information that I have:

Moon rise time
Moon set time
Moon phase (first, full, last, new)

Those information come from AccuWeather API: https://apidev.accuweather.com/developers/astronomyAPIGuide#MoonPhasesRange. There are some other APIs that give the moon phase in degree format or between 0 and 1 but we chose AccuWeather.

I tried to calculate this with only the moon rise and set time. I thought that simply calculating halfway between rise and set time will suffice. However, I didn't always get the same results as the moon overhead and underfoot in this website: http://www.solunarforecast.com/solunarcalendar.aspx

Is there a formula to get the time of moon overhead and underfoot with the information that I have?

UPDATE

I used the definition

  when the moon is directly overhead in the sky

because that is what I found from my research. However, it's seems the correct definition is when the moon is at highest point in the sky.

Also, since it's for hunting purposes, I don't need the time to be accurate to the minute (but that could be nice).


texts: how long a light year on earth will seem

texts:
First of all, sorry if this is not the right StackExchange to ask this question, but i don't know a better place (open to suggestion in case).
So, the question:
Suppose that for some reason, we can travel at the speed of light, and lets suppose that there is no acceleration involved, so it's something like uniform rectilinear motion.
Let's say that today someone leave with this spaceship at the speed of light, the after one year (for him), he will have travelled 9460730777119.56 km, but how much will be spent on the Earth since he has leaved?
So, in other words, I read about a special planet 31 light years from Earth, and I was wondering, if we can travel at the speed of light and someone travels for 31 years to that location, how much older would I be from when he has leaved the Earth?


texts:
I checked an app and saw that today, the sunrise and moonrise time are almost same. The sunset and moonset time are also same.
Why does this happen?
I have always known that moon rises at night and sets at dawn.


texts: Which quantum reality will be seen in a future Earth

texts:
If we orbit near a black hole to travel to future and come back to Earth, which of the quantum realities will be observed in the future Earth?

My question is regarding many worlds interpretation of quantum mechanics according to which multiple alternate futures are possible https://en.m.wikipedia.org/wiki/Many-worlds_interpretation


texts: Estimating the Rate at which Moon is Moving Away from the Earth

texts: Here it has been discussed why the moon is moving away from earth at a rate of 1.5 inches per year. Is there any way we can estimate the value (e.g, 1.5 inches per year) mathematically?


texts: Why is it impossible to estimate the inclination of an exoplanet with the Radial Velocity Method?

texts:
The formula of the radial velocity is:

$$v_i= V - K(\sin(f_i+\omega)\ + e\sin\omega),$$ with
$$K = \frac{m_p}{m_s+m_p}\frac{na\sin i}{\sqrt{1-e^2}}.$$

Being $V$the systematic velocity of the system,
$m_p$ the mass of the planet, $m_s$ the mass of the star, $n = \frac{2\pi}{P}$ the mean motion and $P$ is orbital period of planet, $a$ the length of the semi-major axis of the planet, $i$ the inclination of the orbital plane with the ecliptic,
$e$ the eccentricity of the planet, $f_i$ the true anomaly at time $t_i$ and $\omega$ the longitude of periastron.

It's well known the usual procedure. You have data about the instances $t_i$, the radial velocities $v_i$ at time $t_i$ and then you also have the measurements uncertainties $\sigma_i$. With that data, you can estimate 6 parameters: $V$, $K$, $e$, $\omega$, $T$ (the periastron passage time) and $P$. After you estimate these parameters, you can calculate (if you know the mass of the star and you assume that $m_p<<m_s$), $m_p\sin i$, which is known as the minimum mass. So, as far as this way shows, you got no way to estimate the mass of the planet and the inclination separately.

But what if instead of estimate this set of parameters, we introduce another one? Just one with all the parameters in the equation: $V$, $m_p$, $i$, $e$, $T$, $\omega$, etc.? Couldn't that be possible? But nonetheless, it's always assumed that it's impossible to know the inclination with the RV data. Is there some reason why?


texts: Hypothetical CMB space telescope design problem, received power from extended thermal source versus receiver front end NEP?

texts:
Consider the example of a large radio dish antenna in space equipped with a heat shield protecting it from the Sun. Let's say the amplifiers front-end effective temperature (for purposes of noise calculations (i.e. NEP) is 2.7 Kelvin which is coincidentally the temperature of the Cosmic Microwave Background that it will look at.
We'll ignore thermal radiation from the dish itself, because although it is somewhat warmer it's highly reflective metal and so the emissivity is very low. (see this answer for more on that)
The receiver system is fixed in central wavelength near the peak of the CMB, let's say 2 mm, and has a fixed 1% bandpass.
Now it's time to choose the diameter and focal length of the dish.
I know that the characteristic "sky temperature" of the CMB for which I'm trying to image non-uniformities and the temperature of my receiver's front end are both 2.7 K but what is the ratio of their powers that I'll be measuring?
Question: If I have a large f/no. will a given diffraction-limited feed horn receive less power and so the signal will be weaker compared to thermal noise of the receiver, or will they end up somehow being roughly equal no matter the diameter or focal length of the dish?


texts: Today at 7:24 pm, I saw to stars moving parallel to each other for a few seconds in the sky. Then, both suddenly vanished. What phenomenon is this?


texts: L2 point ephemeris (celestial mechanics)

texts:
I'm a master student and I am trying to get the L2 ephemeris for some calculations in my master's project. It was kinda hard to find a file with the L2 ephemeris, but once I found it, I tried the next syntax.

from jplephem.spk import SPK
kernel = SPK.open('L2_de431.bsp')
position = kernel[3,392].compute(2457061.5)

but I'm getting an exception: "Only SPK data types 2 and 3 are supported".

I tried the same syntax with a planetary ephemeris file (de432s.bsp) and it works fine.

Can someone help me with that or knows any other ephemeris file for L2?

L2 file is here: https://naif.jpl.nasa.gov/pub/naif/generic_kernels/spk/lagrange_point/


texts: How to find star location knowing latitude and LST

texts: Newbie learning Astronomy and was curious how to locate a star giving a Latitude and LST.  For example if my LST is 4:00 and my latitude is 20*N, how would I find where the Horsehead Nebula appears in the sky?  The only formula I can think of is the Hour Angle formula..


texts: I was looking at Saturn through a basic reflective telescope. I had it focused and could see the rings. To the left a small dim white dot came across my view passing in about 3 seconds from my north to south( perpendicular to the Rings) no tail or anything.


texts:
Every day I am learning more about the Sun's atmosphere, recently about the Wilson effect. The 2009 Saas Fee Advanced Course 39 states:

Near the solar limb, the umbra [the circular dark region of a sunspot] and the centre-side penumbra [the lighter border of the sunspot] disappear. We see $400-800{\rm km}$ deeper into sunspots than in the photosphere.

Then, there is a publication entitled The sunspot—shallow or deep? by Solov’ev and Kirichek from 2014 which says in the abstract:

Two sunspot models are compared-shallow and deep. According to the former, a sunspot, as a region occupied by a strong regular field and comparatively cold plasma, penetrates into the solar convection zone to a shallow depth of about 4 Mm. [...]

Furthermore, there is a PhD thesis by Benjamin Beeck from 2015 which shows e.g. the following graph:

In other words, I have values between $400 \ldots 7300 {\rm km}=7.3 {\rm Mm}$ for the depths of a sunspot which confuses me slightly - I would appreciate if somebody could explain me the rather large range of depths. One the one hand, I am interested in the depth of the umbra measured from the penumbra (if it can be seen as flat), and on the other hand I also would like to know the distance from the penumbra to the end of the (visible) photosphere, everything ideally with errorbars.


texts: Would a high albedo reflective substance cool down Venus?

texts: If we were to put a highly reflective substance into Venus's upper atmosphere, could we cool it down?  I am envisioning adding a highly reflective fine dust or gas, possibly lighter than "air" to the atmosphere.  I believe to have the proper effect, it should also reflect infrared light, which causes heat.  Apparently Venus already has a high albedo of .75 (reflects 75% of light): http://www.universetoday.com/36833/albedo-of-venus/ .  However, if we were able to raise that higher, even up to 99%, would that cool it down, perhaps even making it habitable (temperature wise), or possibly even making it ice cold?


texts: How exactly is the Initial Mass Function (IMF) calculated?

texts:
The Initial Mass Function (IMF) is the empirical function which describes the initial masses of a population of stars. My questions are,

1) What are the various IMF's which are used?

2) For each, what type of population do they describe? (e.g. - galaxy, dwarf galaxy, globular cluster, etc..)

3) And how are they actually calculated? (meaning, do they come from simulations/observations, and what assumptions are made about each?)

Entire answers and pieces of answers are all welcome. Formulae (in latex, please) are encouraged.


texts: According to the  Friedmann equations big bang happened everywhere, but if we account for inflation theory, big bang did not happened everywhere. How ? Anyone please explain. Please see the video for reference.


texts: How would an exoplanet be found from earth if our view of its star system is "from the top"?

texts:
When we find an exoplanet by observing the dimming of its parent star, this means that the planet passes in front of the star from our view.

What if our view of a star system is from its "top" or "bottom"? Then from our view, none of its planets would pass in front of the star so we could observe a dimming. How would we know, then, whether such a star has or doesn't have planets?


texts: Is the dark energy between the moon and Earth measurable in any capacity?

texts: What are the experiments, or measurements, that can detect, or account for, dark energy involved in making calculations concerning Earth and its only natural satellite?


texts: Can we observe what objects exist at the L3 positiion of planets of our solar system?

texts: From what I understand Earth's L3 is obscured by the Sun, so is that true for our viewpoint of the other planets? Or, are there times when other planets are on the other side of the Sun from Earth and we can see their L3 position? For the Hilda's of Jupiter have we observed them in that position or is it that we have calculated their coordinates to be in the L3 position?


texts:
Sudarsky's gas giant classification predicts the visual appearance of gas giant planets based on their temperatures.

But what determines their temperatures in the first place?  Is it just the irradiation from their star, or is there more to it than that?  If there's more to it, what else is involved?


texts: How can it be known that Venus does not have plate tectonics?

texts:
This answer provides some insight into Venus' surface geology:

  Water may be necessary as a lubricant for plate tectonics. Whether or not this is the case, Venus does not have plate tectonics. It instead has a stagnant lid geology, punctured occasionally by extreme vulcanism (Siberian traps level vulcanism, and then some).

What are the observations that lead to this conclusion? The existence of plate tectonics on Earth was first determined by putting a lot of pieces of the first-hand observational puzzle together. There's much less data available from Venus.


texts: How wide is the Moon's umbra and penumbra at EM-L2?

texts: Measuring the total width of the Moon's umbra and penumbra at EM-L2, 64,700 km, how wide would the shadow be?


texts:
Which planet is also the name of an element?

I came across this question on cosmos quest and need and answer


texts: How could I determine what areas of the Milky Way will be visible?

texts:
There is a new moon on Saturday, and the forecast in my area of Winnipeg, Manitoba, Canada calls for clear skies on that night.

I would like to take a widefield astrophotograph.

I have no idea what areas of the milky way will be visible, and where they will be located.

What are some steps I could take to determine this?

Are there different dense (photogenic) portions of the milkyway visible at different times of the year?

Are there certain constellations associated with the dense portions of the milky way that I should be looking for?

I would like to further develop my knowledge of astronomy, for the purpose of widefield milkyway shots.


texts: What causes the bright white area in this photo of the earth?

texts:
Regarding the photo of the earth described here,

I know that the green tinge on the left hand side is to do with the aurora borealis. What's causing the bright white light on the right hand side?


texts: What is the gravitational force felt on Earth from the other planets in our solar system?

texts: How much gravitational force is felt on earth from the other planets in the solar system? The sun exerts the strongest g-force, holding us in it's orbit, followed by the moon which affects the tides on earth, but how much force do we feel from Jupiter, Saturn, Venus, etc?


texts:
From the theory of relativity, if any object is moving with the speed of light or more than the speed of light, then the time stops or seemed to be stopped for that object. That means, that the speed of time for an object moving with the speed of light is zero.

So what will be the speed of time for an object in the space, travelling at a negligible speed(i.e.,zero) or approximately stationary.


texts: What are the dark spots in this Hubble image of the Crab Nebula?

texts:
I recently ran into this NASA Hubble image of the Crab Nebula in this article talking about its core. What I am interested to know though is: what are these round dark spots appearing in the image (upper right, lower center and lower left for example)? "Holes" in the Nebula? Objects in front of the Nebula, visible against its illuminated gas? Blemishes in Hubbles lens(es)?

CLARIFICATION: I am asking about the small pixel like dots. Not the large "holes" from this question.


texts: Why don't Neutron Stars form event horizon?

texts:
Trying to compare density of Black Holes and Neutron Stars I came up with the following:

A typical neutron star has a mass between about 1.4 and 3.2 solar masses1[3] (see Chandrasekhar Limit), with a corresponding radius of about 12 km. (...) Neutron stars have overall densities of 3.7×10^17 to 5.9×10^17 kg/m^3 [1]

and

You can use the Schwarzschild radius to calculate the "density" of the black hole - i.e., the mass divided by the volume enclosed within the Schwarzschild radius. This is roughly equal to (1.8x10^16 g/cm^3) x (Msun / M)^2 (...)
The value of the Schwarzschild radius works out to be about (3x10^5 cm) x (M / Msun) [2]

Let's take a neutron star from the top of the spectrum (3.2 Msun) and same mass black hole.
Converting units:

Neutron star: 5.9×10^17 kg/m^3 = 5.9 × 10^14 g/cm^3
Black hole: 1.8x10^16 g/cm^3 x (1/5.9)^2 = 5.2 x10^14 g/cm^3

The radius of the black hole would be (3x10^5 cm) x ( 5.2 ) = 15.6km
The 3.2Msun Neutron Star of this density would have volume of 1.08 x 10^13 m^3 which gives radius of 13.7 kilometers
According to Shell Theorem, spherical objects' gravity field strength at given distance is the same for spheres as for point masses so at the same distance from center of same mass (point - black hole, sphere - neutron star) the gravity will be the same.
That would put the surface of the neutron star below the surface of event horizon of equivalent black hole. Yet I never heard about even horizon of neutron stars.
Either I made a mistake in my calculations (and if I did, could you point it out?) or... well, why?


texts: I know that the Hubble telescope was named after Edwin Hubble, due to his astronomical discoveries of astronomy, but who designed it? Which people were most directly responsible for the inspiring and planning the HST project.


texts: What is the temperature inside a Black Hole?

texts:
Is it insanely hot? Cause of so much gravity?
Is it close to absolute zero? Cause the matter is so closely packed, there is hardly any space for particles to move?
Is it room temperature? Cause Cooper didn't die in Interstellar when he went into a Black Hole?


texts: How is the Universe's Expansion Accelerating if the Hubble Constant is Decreasing?

texts:
I am just getting into the field of cosmology and was wondering what it means practically to say that the Hubble Constant is decreasing while the expansion rate of the Universe is accelerating. I am not sure how this phenomenon is possible. How do we observe this?

It is my understanding that the Hubble Constant is the rate at which the Universe is expanding, and it is decreasing with time. (Although it is asymptotic)... Is this conception flawed?


texts:
For Earth, the lunar month is well known. But Earth has only one moon.

But on Jupiter, how long does it takes for the same moon configuration to appear in the "sky" of the planet?

For the sake of sanity, you can consider only the four Galileian Mooons.


texts:
I know not all stars can "go nova", and that of those that do there are different types of novae, among which are some that aren't even explosions (at least not as we think of them), and that even the ones that are explosions are really not like what we do with TNT or C4 anyway.

Nonetheless, I can't shake the comparison from my head, and that got me thinking: Are there any circumstances under which one star going nova can trigger novae in other nearby stars, creating a chain reaction?


texts: Could the word "multiple stars" include binary stars?

texts:
We are translating some illustrated book for kids about astronomy, and then have a question as in the title.

Referring to Wikipedia etc., It appears that "binary stars" represents the star systems of two stars, whereas "multiple stars" does those of three or more stars.

Thus, our question is whether it is an abuse to use "multiple stars" for binary stars.

Thank you in advance.


texts: Can a star eject a liquid or a gas that has not been ionized? I am implying that these gases could be a source of reusable energy.


texts: How clearly would I be able to see a galaxy with the naked eye if viewed from a "close" distance?

texts:
In images of other galaxies they look very bright and vivid against the blackness of space (example). If I was located a few tens of thousands of lightyears away from a galaxy, would I see something similar or would it be more like a slightly brighter smudge in space?

I'm wondering since it seems we can barely see the features of our own galaxy with the naked eye from earth.

Edit: I see that someone has downvoted the question, it would have been good to have some feedback! I realise the question might be a bit vague, but I thought it was interesting to know if galaxies would look as bright as we see them in the pictures.


texts:
How to resolve confusions on the rubber sheet analogy of the spacetime curvatures?
I am a newbie to spacetime curvature. I have watched several youtube videos on Einsteins GR and spacetime curvature where most people used the analogy of the rubber sheet and ball on the sheet. However, I cannot realize that rubber sheet analogy with my reality. For example,

The rubber sheet is a two dimensional sheet on the other hand I see our universe is a three dimensional.
The balls on the rubber sheet are placed from one direction to other (top to bottom) but in reality I see out planets and stars are flying on the space.
Light flows over the rubber sheets and curves around the balls in its world line, but in reality we know everything themselves are travelling with the speed of light in their world line.

How can I resolve these confusions? I am really interested to know detail about them.


texts: Does the Moon have any oxygen in its atmosphere?

texts: Since the moon has gravity, it's almost impossible that there aren't some gasses trapped on the surface by the moon's gravity. Has any free-floating oxygen been found on the Moon? If so, in what concentration?


texts:
Earth (and all of its 'neighbors') is a spheroid. Why is this?

Earth could've been a cube, a tetrahedron (pyramid) or really any shape! But why a sphere?


texts: Is there any phenomenon where a supernova doesn't anything leave behind?

texts: I have been researching this on google, and I think there is a process. What is this process called? I have forgotten.


texts: Is dark matter influenced by gravity of baryonic matter?

texts:
As far as I know galaxies and clusters form along so-called "galactic filaments" which are influenced by a dark matter web that organises them in to this position in the universe. So it looks like dark matter attracts baryonic matter by a kind of gravity. But is it also true the other way around, and that 'our' mass has gravitational forces on dark matter?
And if not why does it only go one way?

In a study scientists have observed the four colliding galaxies of cluster Abell 3827 and discovered that one of the envelopes of dark matter in the galaxy behind it encloses. The backlog in the system stands at 5,000 light years (50,000 million million kilometers - the NASA space probe Voyager would do 90 million years to cover that distance.
Expected to create some lag between a galaxy and its associated dark matter when dark matter affects itself, however small, by other forces than the gravity.

See also https://www.youtube.com/watch?v=qtfTUPhXutQ and
http://spacetelescope.org/news/heic1506/


texts: Free neutrons and stellar nucleosynthesis

texts: Both the R-process and the S-process of stellar nucleosynthesis rely on absorbing 1 (S) or more (R) neutrons. Where do they come from? I thought there are no free neutrons in stars. (Also the half-life of a free neutron is only 15 minutes, so the source has to be inside the star)


texts: Is "Zero Shadow Day" necessary to perform Eratosthenes' experiment?

texts: Eratosthenes experiment to find the circumference of the Earth is usually said to be performed on the Zero Shadow Day at Syene. Is it necessary? Can this experiment be performed on any day for two locations lying on almost same longitude?


texts: Where/how are metals distributed in our universe?

texts:
For one particular kind of element, what is the distribution in the whole universe?

Are most of the metals located in ISM, IGM, or stars? Maybe different metals have different distributions?
For example, could most of the Mg be found in white dwarfs?
Have all the natural elements have been found in stars?


texts:
My telescope is Arsenal - Synta 70/700 AZ2 (refractor model). 140x max zoom (10 mm with Barlow).
I could clearly see Jupiter and Saturn in color. I even saw brown stripes on Jupiter.
Yet, when I tried looking at Venus and Mars they appeared pure white, totally washed out by the Sun. I was observing about hour before sunrise.
Is there a way to see them in true color with my current tech? Maybe they could be seen only at certain dates? Or maybe I need some filters?


texts: How early could life supporting planets been formed?

texts:
In thinking about exclusion options for where its not worth to look for habitable planets, the past came to my mind.

Right at the beginning of the universe, there was no possibility for (carbon based) life supporting planets to form, simply because there was not enough material to do so.

Population III stars burnt out within millions of years to leave some of these elements, but was it enough to form planets? I have some problem getting things into perspective, considering that

there are so many processes that produce different kinds and amounts of isotopes
these isotopes are spread out in various different ways (and thus also different concentrations from how they were created) throughout the interstellar medium
it takes probably a lot of time to gravitationally condense enough metals into a planet once it was spewed out from e.g. a super nova
the majority of stars are smaller so it would probably take a clustering of heavier stars to provide the conditions earlier in the universe.

I would consider a similar distribution of metals like in earth as sufficient for life, and would concentrate only on those found in DNA (N,H,C,P,O), fats (C,H,O) and proteins (C,O,H,N) as well as most of the essential minerals (Ca,Cl,K,Na,Mg,P,S) and maybe at least a bunch of (possibly essential) trace elements (Co,Fe,I,Cu,Mn,Mo,Se,Zn,As,B,Cr,F,Rb,Te,V,Sn,Ni).

So what is the best approximation that we can currently give on when these elements were abundant enough to condense into earth like rocks with earth like elemental distribution?


texts: What is the orbital period of our sun around its barycenter?

texts: In years, what is the time it takes our sun to orbit around our solar system's barycenter?


texts: Will Sirius B start accreting from A and become a supernova type Ia?

texts: Sirius B is a massive white dwarf of 1 Solar mass, orbiting at about 25 AU distance from the 2 Solar mass Sirius A. As it evolves and expands, will the A star start shedding matter to the white dwarf, and when will this start happening? Will the Sun be at a safe distance when/if it does happen, or is Sirius our doom?


texts:
After about 34:00 in the 9th press conference of AAS 235, radio astronomer and NRAO's spectrum manager Harvey Liszt talks about Radio Astronomy in a New Era of Radio Communication and says:

This is a new development that we’re going to have to come to grips with. There is a band allocated between 9.2 and 10.4 GHz (this is roughly an order of magnitude lower than you saw for CloutSat) and these are being developed commercially now, and if you look on the right you’ll see ICEYE. ICEYE’s slogan is “Every square meter, Every hour”.
A synthetic aperture radar like this, if pointed at a radio telescope when the telescope is pointed in its direction will burn out the radio astronomy receiver.
As a matter of fact, some of these radars are so strong that even the ground scatter (the albedo of the ground is about 10%) and even the backscatter from radar will burn out a radio astronomy receiver.

From The BBC's SpaceX Starlink mega-constellation: 'Limited time' to fix brightness issue

One of the radar constellation companies named by Dr Liszt as posing a potential problem is Finland's Iceye.
The firm's CEO, Rafal Modrzewski, said his team took its responsibilities seriously, and that it continued to investigate any potential risks Iceye's technology might pose to the global space community, both in orbit and on the ground.
"Most importantly in this topic, Iceye complies with international regulation regarding our allocated bands," he told BBC News.
"Mitigating any potential or perceived risks to sensors on ground can additionally be done proactively by tracking applicable SAR satellite orbits to avoid clashes with instrumentation, and in the case of Iceye, by also working with us if there are any remaining concerns."

See also the the discussion of the impact on radio astronomy in The BBC's Capella Space radar company chases persistent vision

At the recent American Astronomical Society meeting in Hawaii, Dr Harvey Liszt from the US National Radio Astronomy Observatory raised the concern that powerful radar pulses from orbit could damage the radio receivers that scientists employ in their ground observatories.
(Capella's CEO) Payam Banazadeh told me Capella was taking this matter very seriously and was keen to co-ordinate his company's activities with the AAS.
"We have also been in contact with Dr Liszt and are actively working to safeguard their mission when operating our SAR constellation.
"Whether it is observing Earth or deep space, Capella believes in exploration and scientific discovery. We also believe in and support protecting the mission of the radio astronomy community."

Question: How do radio astronomers avoid having their receivers burned out by ground-imaging radar from satellites?

Do they keep track of known satellites and "avert their eyes" by steering dishes away from expected trajectories?
Is this planned during a scheduling phase or during an observation?
Would this kind of interruption and movement be detrimental to the quality of data for a long observation?

Video: AAS 235 Press Conference: Astronomy Confronts Satellite Constellations


texts:
The Earth revolves around the Sun and the Moon revolves around the Earth.
Out of curiosity I started thinking about the orbit of the Moon around the Sun and expected (assumed) it to be as follows:

But on Wikipedia and some other sites I found out that the orbit is actually like this:

I have 3 questions:

What is the reason for this difference between assumed and actual path variation?
Has this path been like this since the formation of the Moon?
Do natural satellites of other planets also follow the same orbit around the Sun?

After further searching I found out a better, easier explanation regarding the orbit path on YouTube for those interested, be sure to check it out.


texts: Fractional contribution of secondary star

texts: What do we mean by "the luminosity fraction" and "the fractional contribution of the secondary star"  of binary stars? can anyone suggest any method to calculate them theoretically?


texts: If I get the numbers right, at about 17 Jupiter masses a mainly gaseous planet would ignite into some type of star. What about rocky planets? Is there a theoretical limit to their size, so that they can grow up to size x? (I'm imagining a "liquid" planet would congeal into rock after certain gravity value, and would somehow fit into the "rocky class" as well).


texts: How to determine the eccentricity, knowing the orbital speed and distance

texts: How to determine the orbital eccentricity of a planet, knowing the instantaneous orbital speed of it and the distance from the Sun to the planet at that moment?


texts:
I have just witnessed 2 stars change positions from above the dominant star to the east of my location. Stratford-upon-Avon
They moved off 1 by 1 and drifted away perfectly parallel to each other then disappeared. How.
This is the second time in 2 days that different stars have moved.
This happened at 8.15pm.


texts:
I am computer scientist and now I'm working with some astrophysical projects. During this work I accumulated some questions and maybe here I would be able to find some answers or advice where to look for them. I already read some of the wiki pages on topic, but it didn't help.

We are working with fitting light profiles of some eliptic galaxies with GALPHAT using Sersic and Sersic Exponential models.
1. So first question would be, what exactly means to fit a light profile? I get a bunch of parameters as output (X, Y, Mag, re, n, q and others), but what exactly does it mean?
2. And the second question is what is the difference between these two models?


texts: Calculation of day lengths of moons or moons of moons

texts:
Is it possible to calculate the day lengths of moons or moons of moons? If so, does the day length depend on the longitude as well as the latitude? Also, if we calculate the day length, is it possible to take into account the duration the sun is eclipsed by the parent body?

A bit of a background. I am playing a game, Elite Dangerous, and the game provides basic orbital information about planetary bodies like orbital period, rotation period, axial tilt and some more. I was planning to build a tool around this but I have no idea how to calculate such a thing.


texts:
In this video, between 4 and 6 minutes, the lecturer describes M type stars as being less massive, smaller, and redder than G type stars.

He then gives Betelgeuse as an example of an M type star and mentions that the sun is a G type star.

However, Betelgeuse is more massive than our sun, and larger than our sun.

How can I reconcile these facts?


texts: How to reduce star trailing when taking long exposure photographs?

texts: My telescope's sidereal tracking is lousy: it just doesn't work. To overcome this, when taking long-exposure photographs through it, what's the best way to reduce star trailing?


texts: Energy deposition in planet Earth due to gravitational waves

texts:
The gravitational wave detected by the LIGO observatory acted on planet Earth streching and shrinking it a little bit. I assume that not all of this action was fully elastic and that some energy of the gravitational wave was lost by the passage through planet Earth.

Are there some scientific estimates of how much energy was deposited in planet Earth by this gravitational wave?

Are there estimates how much energy is in total deposited by gravitational waves in planet Earth per year?


texts:
Many people say that stars twinkle because of Earth's atmosphere. However, the atmosphere is still there when we look at planets and don't see they twinkling.

Is there a clear scientific consensual explanation for the twinkling of some objects but not for others?

Note: just as a metter of distinction, my question is different from this one. In fact, the moon is obviously different from stars, while my question is based on the fact that the only actual difference between stars and planets is that planets move across the sky, differently from stars (since they are the sky's reference). Proof of my words is the late distinction of Uranus as a planet due its extremely slow motion, although it also doesn't twinkle.


texts: How to calculate the orbital velocity, the high-orbit (quasigeostationary) holding force and the shadow of the disk on Earth?

texts:
[]

The following data are given:

•   The circular orbit of the disk D (quasigeostationary orbit) parallel to the geostationary orbit.
•   Located 4,505 km to the north of the equatorial plane, above the 45 degree parallel. The radius of orbit with respect to the gravitational center of the Earth 41,922 km (calculated by the uhoh we thank)
•   Disc diameter: 10 km.
•   The disc consists of a support frame and solar panels. The specific weight approx. 4.5 kg per square meter.
•   The time when the disc makes a complete, retrograde motion, around the earth: 25 hours.

Required:
1 Orbital speed of the disk so as to achieve a complete circular motion in 25 hours.

2 The force required to  maintain the disk in orbit and to compensate gravitational attraction.
2.1 How can this force be assured?
2,2 with thermochemical propellants (rockets with liquid fuel that could be started periodically?
2.3 electromagnetic, ionic, photonic propellants with cold gases.
2.4 And so on

3   What is the diameter of the disk shadow on the Earth's surface?

Figures 1: 2 and 3 graphically reflect the problem.

Any critical comments or comments are welcome.

enter image description here
enter image description here


texts: What causes "North-South ambiguity" when doppler radar imaging a planet surface equator on?

texts:
I'm having a hard time understanding why there would be this ambiguity. Also, why would this be not the case if the planet was viewed pole on?

Edit: "NOrth SOuth ambiguity" is a term used in a paper that seemed most concise but I guess not. When using radar to doppler image a planetary surface which happens to be viewed equator on, we have no way of distinguishing whether the surface feature read in return signal belongs to the northern or southern hemisphere. WHat causes this?


texts: How to convert apparent visual magnitude to SI units?

texts: How do I convert the apparent visual magnitude of astronomical objects to SI units (lumens)?


texts: What is the furthest object from which fermion rays were detected?

texts: What is the furthest object from which non-electromagnetic cosmic rays were detected?


texts: Does the orbital decay of Triton affect Neptune's rotation?

texts: Just a quick question regarding Neptune. Because Phobos is spiraling in towards Mars, Mars' rotation speeds up. Does the same thing happen with Neptune's rotation?


texts: Can someone please tell me what the very bright star is that I see throughout the evening in the northern sky?

texts: I'm located in NYC. I know it's not a planet, although it is very spherical and looks like one. Perhaps it's a satellite, because it doesn't move much and I don't recall ever seeing a star that appears so round.


texts: How does angular resolution of a telescope translate to its parallax precision?

texts:
We can often read in the scientific and also more casual reader literature and articles about the angular resolution of various telescopes and other optical equipment, be it ground based or onboard space probes. They would often list their angular resolution, or in other words, their ability to resolve or distinguish small, distant objects with today's digital era mostly on a per sensor pixel basis.

                Finding a star's distance from its parallax. Trigonometric Parallax method determines distance to star by measuring                its slight shift in apparent position as seen from opposite ends of Earth's orbit. (Source: Measuring the Universe)

What I'm interested in is, is the precision in measurements of parallax and with it our ability to determine distance of observed objects directly analogous to mentioned radial resolution and how could it be calculated using data on angular resolution of a telescope alone, if we assume both ground based and in space observatories have more or less the same perihelion to aphelion distance (i.e. the space observatory is in Earth's orbit).


texts: Can a person at a photon sphere of a black hole decide where the black hole is?

texts:
A person at the photon sphere of a black hole will observe the following:

The black hole surface will cover exactly half of the visible sky (say, the left half), and cosmic horizon will cover the other half.
Looking in any direction between these hemispheres the person will see oneself.
The person will observe no centrifugal force however fast he moves. Any object the person thowing to the left will experience the centrifugal force pulling it further left, while any object thrown to the right also will experience the centrifugal force pulling it to the right. Approaching the black hole in such circumstances may look quite like actually escaping it to the crew.
The space to the right as well the space to the left seemingly includes objects of any length and dimensions, the further objects are, the more they are time-dilated and length-contracted.

Is there any way for a spacecraft commander to decide (by an experiment conducted in a small$^1$ amount of time) which hemisphere is the sky and which hemisphere is black hole in such circumstances?

--

$^1$ Here small means small compared to characteristic time scales of the problem.


texts: Does anyone have spectra on exoplanet HD 219134b? How could one know either way?

texts:
I have foraged through the internet to no avail.
Is there a way to systematically check if spectra from a given exoplanet has been reported, like a catalog or list of references, or is a literature search using likely keywords the only way to look into this? Thanks!


texts: Can black holes be a subset of quark stars? If not, what are black holes made of?

texts:
Can stellar black holes be a category, or subset, of quark stars?
<img src="https://i.stack.imgur.com/QpE2v.jpg" alt="
" />

Strange stars are so called not because they're weird (though they
are) but because the extraordinary pressures within them should turn
some of their constituent quarks into a heavy type of quark named
"strange." Neutrons and protons are made of two lighter types called
"up" and "down." (The names were chosen whimsically and have no
particular meaning.)
"It's possible that all the stars we have been calling neutron stars
are in fact strange stars" or hybrids of the two, said Turner. If
strange matter does convert everything it touches into more strange
matter, these stars would consist of the stuff right up to their
perfectly hard-edged surfaces, with nothing but vacuum above. If it
doesn't, the stars could be hybrids with strange-matter cores and
neutron-star outer layers.
from https://skyandtelescope.org/astronomy-news/quark-matter-stars-said-found/

The visible quark stars such as the one depicted would be quark-neutron hybrids. This would conveniently answer the question "what is black hole substance made of".
If this is certain to not be the case, what (if any) known substance can black holes de made of?
Note, I said a subset - "...strange stars exhibit a low-luminosity but high-temperature bremsstrahlung spectrum, which, in combination with the emission properties of the accretion disc, may be the key signature to differentiate massive strange stars from the black hole." in https://academic.oup.com/mnras/article/400/3/1632/962273
https://physicsworld.com/a/calculations-point-to-massive-quark-stars/


texts: Could gravity waves be used to confirm the existence of a ninth planet?

texts: Shortly after the news of the discovery of gravity waves, my physics professor explained the significance of the discovery by comparing it to a medical X-ray machine. He said that gravity waves can be used to detect objects that cannot be seen using telescopes. Therefore, could gravity wave detectors be used to confirm the existence of the theorized ninth planet? Even though it doesn't have a high enough albedo to be seen, it would still be detectable gravitationally, wouldn't it?


texts: How are various orbital period measured?

texts:
How are the sidereal periods and synodic periods of celestial bodies measured from earth? The usual definition of the sidereal period of a celestial body is the time it takes to complete one revolution around the sun, relative to some other fixed point outside the solar system. I assume that relative to a fixed point here means the references frame of that point.
Is there a way to measure this sidereal period while still staying in the reference frame of the earth? For example, a sidereal day can be measured by noting the time between the first appearance of a distant star on two consecutive nights.
Similarly, what are the ways for measuring synodic periods?


texts: Dark matter emitting EM radiations beyond our currently known spectrum?

texts:
Could it be possible that dark matter is emitting EM radiations on a scale that is not known or perceivable by humans yet and undetectable by man-made instruments?

A type of radiation that our instruments were just not designed to pick up on? Being insensitive and not advanced enough? Thus we do not yet have all the pieces to solve the puzzle?

Edit: We need to remember that there are infrared and gamma rays on either end of the spectrum and we are aware that dark matter is definitely not emitting visible light, or any of the other type of EM waves. Considering that gamma rays were discovered roughly only a hundred years ago, isn't it possible that the spectrum could be further extended on either side and that we're still not aware of it?


texts: Is the Drake equation based on a logical fallacy?

texts:
I often wonder why the Drake equation gets taken so seriously and why popular scientists in general say "it's probable' life exists elsewhere in the cosmos.
My problem is that the mechanism for abiogenesis is as yet unknown.

We don't know the likelihood that RNA like molecules can assemble from chance or for that matter that this is the source of life on this planet (as opposed to coming via a comet). We do know that we have no record of it recurring and as yet Miller-uri style experiments have not yielded results.

The argument I keep hearing from Dawkins, Sagan and many others is that the universe is vast and life occurred relatively quickly in our planet's history which suggests it is common.
I detect a logical fallacy based on the inductive reasoning of using one occurrence in order to establish a pattern.

Think of it this way. If I've never played cards and get dealt my first hand which is a royal flush I might think its quite likely based on my past hand that I will get another in the next deal. This doesn't change if the deck contains a hundred suites and the odds of a royal flush is astronomical - if my first hand is a royal flush.

Similarly the vastness of the cosmos tells you nothing about the likelihood of other life if you don't the probability of its occurrence.It could be very likely or so rare as to only occur once.
Does anybody have an opinion on this?


texts: Clarification on calculating IR Dust Extinction $A_K$/$A_V$

texts:
So I'm calculating V_Alf Ori's Luminosity (or Betelgeuse) using Davies et al. (2013) SED fitted formula from Near-Infrared/NIR K-Band magnitudes and it requires the subtraction of 'extinction'. However, the conventional prescriptions for extinction in the NIR K-regime is a little way off for a massive star with a very dusty environment (likely thick as well due to stellar mass loss and winds enriching it). So instead of using those, I may need to do a quick approximation of it.
This maybe a rookie stuff but as I scour in, which needs some quick clarification or validation, I kept seeing this "$A_K$/$A_V$" which I presume came from this one?
$\frac{A_λ}{A_V}$ = $\frac{R_λE_{(λ-V)}}{R_VE_{(B-V)}}$
where;

$R_V$ is the canonical Cardelli redenning value of 3.1.
$A_V$ is optical extinction.
E(B-V) is conventional color excess.
While the remaining upper variables are 'almost' the same depending on the bandpass of choice λ.

Edit (1): Is that 'correct'/applicable or there's something else on that like applying power law etc.?
Edit (2): Already got the idea of $\frac{A_λ}{A_V}$. Then how about $A_K$?how can I get that?


texts: Roche Limit for Earth-like body around Earth-like body

texts:
In a discussion elsewhere about binary planets, it came the question.

How near can a planet like Earth (maybe Earth itself) be to another planet like Earth before both of them tearing apart due to tidal forces?

In other words: Which is the Roche limit for Earth, if it were orbiting (another) Earth?


texts: I've seen that a large block of Pan-Starrs data has been released and is available on-line and how the distributors of the data expect professionals and the general public to use the data in their research. I am part of the general public and was wondering how accessible the data actually is?  For instance if i were interested in searching a small section of sky for a moving object, i would need multiple "photos" "data sets"? over time in order to see the movement.  Would special software be required? How would one even start the process?


texts: Gravity is the result of the curvature of space, so could the curvature actually send objects away from the source?


texts: highspeed black holes or neutron stars on (almost) head-on collision course and kinetic energy

texts:
First: I don't think this is a duplicate question. There are related question on mergers of black holes that orbit around each other etc., so please read on.

I'm having a hard time picturing what might happen in the following scenario:
Assume two black holes of the same size on an almost frontal collision course with a very high relative speed of x km/h (replace x with whatever would be necessary to enable the scenarios described below).

If their event horizons don't "touch" I'd assume their courses will change considerably but it would be a normal flyby - right ?

But what happens if their event horizons "touch", if their event horizons "overlap" just a few meters ?

On the one hand both objects have a massive kinetic energy with opposite directional vectors. And on the other hand nothing can escape below the gravitational radius.

Where does all the kinetic energy go ? Would the two black holes extremely deform, like creating a long string before they collapse into spherical form?

And what if in a similar scenario one of the two objects is a neutron star. Would the neutron star be ripped apart when it scrapes the black hole on high-speed flyby ?


texts: Do white dwarfs lose mass as they fade to black dwarfs? Is there a correlation between temperature, mass, and radius?

texts:
I'm having a difficult time understanding certain behaviors of white dwarfs.

I understand how mass is lost in the red giant to white dwarf transition process. I understand that white dwarfs can accumulate mass from a partner binary or some other source and I'm aware of the 1.4 M☉ Chandrasekhar limit. I get that they're not losing all their mass; in general they're just cooling and in the end there will be a lone black chunk of crystallized material floating forever in the void. I know they are really insanely hot when formed and very cold after an unfathomable amount of time. But is it that same, initial mass, or does it somehow diminish?

But is that initial mass the mass the remnant has after its cooled to a black dwarf? I guess what I'm asking is that I'm trying to figure out if there is a correlation between temperature, mass and radius. Some sort of equation?


texts: Does the Sun have any atmosphere of its own? Is it just like the atmospheres that the planets have.


texts: How to view Jupiter with 4.5" Newtonian Reflector

texts:
I have a 4.5" Newtonian Reflector and I took a look at Jupiter the other night.

I was able to get a reasonably sharp view of it and see a number of it's moons, but I was unable to see any surface details, it was pretty much just white glare.

I'm not sure if it's possible to distinguish any surface details with the telescope I have, so any advice on best setup would be useful.

I have a few accessories:

x2 barlow lense
10mm eye piece
25mm eye piece
6mm eye piece
moon filter
color filters

Would anything else, other than a more powerful telescope, help me?

I was unsure if I might have an alignment issue that was causing this, but as I could make out the moons as sharp dots on the planetary plane, I assumed not.

Any advice would be welcomed.


texts:
It has been suggested by some futuristic or sci fi leaning thinkers, that Ceres' surface might be mined for water to support human exploration and settlement of space. But NASA's Dawn mission and other observations show that it is very dark and seems to consist of "hydrated minerals" in an environment warm enough to make water ice sublimate. Does this mean that mining water on Ceres is really hard? Requiring deep drilling or excavating through thick hard surface layers before useful water ice is encountered.

Could a lander on Ceres just heat up the surface and collect water sublimating from it?


texts: Does the expansion of the universe increase with distance?

texts: I read something in a book the other day which has gotten me really confused. They were talking about the expansion of the universe, and if I understood it all, they said that the expansion of the universe increases with distance. I’m wondering if I understood this correctly, because I was reading about Hubble’s law and it said that the redshift if galaxies far away (caused by the expansion of the universe) is proportional to the distance (up to a few hundred megaparsecs away). Doesn’t this mean that the expansion is the same but the redshift increases? (I don’t see why the redshift would be proportional and a linear graph if the universe expanded from us proportionally to the distance, or at least more further away). I hope you understand what I mean by all this.


texts: Did nobody in the Astronomy community think 12,000 new satellites in LEO might be a problem?

texts:
This answer to Can / should the Starlink satellites be blackened? mentions Space News' SpaceX working on fix for Starlink satellites so they don’t disrupt astronomy where SpaceX chief operating officer Gwynne Shotwell is quoted as saying the following:

Shotwell admitted that nobody in the company anticipated the problem when the satellites were first designed.
“No one thought of this,” she said. “We didn’t think of it. The astronomy community didn’t think of it.”

Question: Did nobody in the Astronomy community think ahead of time that 12,000 new satellites in LEO might be a problem?
Did SpaceX sneak this in under the astronomical observer community's nose by starting "small" (SpaceX's 4,425 satellite constellation - what's the method to the madness?) and then "drifting" up to 12,000 satellites without anybody realizing it? Or did the community actually have some good idea that this was likely going to be a problem?

fyi it may not stop at 12,000:
Space News: SpaceX submits paperwork for 30,000 more Starlink satellites

WASHINGTON — SpaceX has asked the International Telecommunication Union to arrange spectrum for 30,000 additional Starlink satellites.
SpaceX, which is already planning the world’s largest low-Earth-orbit broadband constellation by far, filed paperwork in recent weeks for up to 30,000 additional Starlink satellites on top of the 12,000 already approved by the U.S. Federal Communications Commission.

SpaceX could one day operate 42,000 Starlink satellites if it builds and launches 30,000 in addition to the 12,000 for which it already has FCC approval. Credit: SpaceX


texts: Why is the difference between Apollo and Aten asteroids considered meaningful?

texts: I understand the difference between the two to be that they both cross Earth's orbit, but Atens have an orbit smaller than Earth's, and Apollo have an orbit larger, but I don't understand why they aren't simply lumped together in one category.


texts: Which binocular should I buy to be able to see Saturn and its rings?

texts:
I am new to Astronomy and never had a binocular to look at the sky, I read somewhere that already with a 30x optical zoom should be possible to spot Saturn and see its rings, is that true?
I have a budget of up to 400 bucks, which binocular would be the best in this range to see Saturn and other objects in the night sky?


texts: How bright can white dwarf stars glow as they accrete matter?

texts:
I realize this is kind of general as it would depend on the size of the white dwarf and the rate of accretion. The general idea I got thinking about is what would happen if a white dwarf star - lets say 0.8 solar mass, well below Chandrasekhar was to accrete a bunch of hydrogen rather quickly.  I thought it might be get very hot and bright, perhaps similar to a red giant for a relatively brief period of time.
Mostly White dwarfs add mass by siphoning from the outer layer from a near by co-orbiting star, but I imagine, that mostly happens quite slowly, not fast enough to make the white dwarf shine like a main sequence star.
Are there studies on how quickly mass can be added in this way and how bright it makes the white dwarf?  Does Hydrogen quickly fuse on the surface of a white dwarf or does it need to build up to a certain thickness first, perhaps creating a mini flash? or does the fusion happen rather quickly.
A very rough calculation, the escape velocity of a white dwarf, which would be similar to the terminal velocity of matter (mostly hydrogen) falling into it is about 5,500,000 meters/second for a 1 solar mass white dwarf.  Source.
The velocity needed for hydrogen fusion is about 20,000,000 meters per second.  Source (under the what makes fusion hard section).   So, white dwarf's aren't massive enough to create fusion upon impact, but the high density and high heat of impact seems like it could create fusion relatively quickly as matter builds up around them, perhaps creating something that perhaps glows like a star and expands rather rapidly - assuming there's a sustained burn and not a kind of very quick explosion of sorts.   I think it could glow very hot and perhaps expand rather large, but I'm just kind of guessing.
If a white dwarf was to crash into, say, a brown dwarf or very low mass star, hydrogen rich, say about .1 solar mass?   Such a brown dwarf would be much larger than the white dwarf but also quite a bit lighter, probably light enough to get penetrated completely though the impact would also likely create fusion and it's difficult for me to imagine what would happen with so violent a crash.   It seems to me that the density of the white dwarf, if it could retain most of the matter from the brown dwarf would be sufficient that you'd be left with a fast burning light weight fusion star, with a very dense core.   This might be an impossible question, but how quickly would a .8 solar mass white dwarf consume the hydrogen from a .1 solar mass brown dwarf, should this rare kind of collision happen.   Would it actually form a star-like object for a period of time, say a million years or so, or would the enormous energy of impact basically blow it apart no new star?  Perhaps a near miss and an inside the Roche limit approach would be better than a direct impact for new star formation.
Thanks.


texts: If the Sun disappeared, could some planets form a new orbital system?

texts:
If the sun were to suddenly disappear, the planets would continue to travel tangentially to their former orbits. (This I know from this answer to a somewhat related question here.)
In such a scenario, is it possible (however unlikely) that a new orbital system would form from two or more of the now-rogue planets?  For example, is it physically and mathematically possible that Earth might end up getting captured by Jupiter and thereby become one of its moons?  Or do the size and speed of the planets completely rule out such a scenario, even if the sun disappeared at such a time that the planets' new trajectories would cross?


texts: Is there a limit to how hot an object an get?

texts: Temperature is a measure of the average kinetic energy of particles in a given place (correct me if I am wrong) so there must be a definite limit to heating like there is an absolute zero to cooling. And what process could achieve this temperature?


texts: What's the difference between the Julian and the Julian Ephemeris Day?

texts:
I'm making a code where I calculate the sun position in a given time. But I also want to learn astronomy with it. I'm following a .pdf to make the program, and in this .pdf it says

  The Julian Day (JD) is calculated using UT and the Julian Ephemeris
  Day (JDE) is calculated using TT.

Is it the only difference?


texts: Are circumbinary and noncircumbinary habitable zones mutually exclusive?

texts: Could a binary star system have an earth-like planet orbiting star A, a second earth-like planet orbiting star B, AND a third earth-like planet in a stable circumbinary orbit? Or does the space that is required between the two stars to allow for stable habitable zone orbits for the first two planets eliminate the possibility of the third planet being both habitable and in a stable circumbinary orbit?


texts: Do you know of any other data analysis algorithm that fits 2-D analytic functions to galaxies and point sources directly to digital images, other than GALFIT? Especially to subtract lens light from strong lensing images?


texts: What would happen if someone had a telescope and watched Betelgeuse when it goes supernova?

texts:
Would that person go blind?

Neutrino detectors and the abundance of Neutrinos would detect the upcoming visible show about 3 hours before any visible signs, so there would be time to point certain telescopes that could handle the brightness towards it.

I'm curious if an individual with a telescope pointed in that direction would have an unpleasant surprise.    Would the scientific community be wise to not announce the massive stellar explosion until after it's visible to avoid potential negative effects from over-eager amateur astronomers.

I realize this is a kind of silly question and it might depend too much on the telescope, but I'm curious.


texts: Why do Auroras only occur in polar regions?

texts: Why is it that Auroras only occur in the North and South poles. Why don't they occur in the equator or anywhere for that matter?


texts: How close to circular is the Earth's equator

texts:
Earth's orbit is about 99% circular. How circular is Earth around the equator? I know it bulges around the equator and is a spheroid. I also know it is not smooth and has oceans and mountains and all that.

Is there a similar measure of eccentricity to describe how circular it is around the equator?

Is it stretched into an ellipse by the sun/moon, and if so by how much?

Just for comparison, does the Earth's equator deviate from a circle more than the Earths' orbit around the Sun deviates from a circle?


texts: Will James Webb see Population III stars?

texts: I have heard that James Webb will see the first stars that our universe produced. Can I assume that we may see galaxies that are so young that all of the stars in them are population III?


texts: Are there any studies that prove/disprove the Oscillating Universe Theory?

texts: Is there any evidence that supports or disproves the Oscillating Universe theory? What kind of observations could provide this evidence?


texts: Need Simple equation for Rise, Transit, and Set time

texts:
I've been looking, unsuccessfully, for hours for a simple set of equations:

Input:
RA and Dec of an object
Observers Lat and Lng
Current Time

Output:
Rise Time
Transit Time
Set Time
(and, hopefully) max elevation at transit
(and, hopefully) current alt and az of object

It seems like this should be a simple relationship, but I haven't been able to find it.
If an equation simply gives the current alt/az for an object, I guess I could solve for az and then find the maximum for a given time range.

Questions:
1. Anybody know of such equations? Or, even better, some sample code.
2. If not, can anybody point me in the right direction?

Thanks


texts: Super massive black hole mass estimation in literature

texts: What is the most accurate method for SMBH mass estimation? In the literature I could find 5 methods : (1) Marconi & Hunt 2003, (2) Hu 2008, (3) Gultekin et al. 2009, (4) Graham 2007, (5) McLure & Dunlop 2002. Please tell me which is more accurate? Also, can you suggest other methods and concern references? Thank you.


texts: Is there enough light to grow plants in the ocean of Europa?

texts:
What is the underwater temperature of Europa?

I know Jupiter emits infrared light but does it emit any light able to support plant life of any type on near by Moons? Can under water volcanic activity of Europa produce enough light to support plant life?


texts: Any studies of the detection of dark matter in or near the solar system?

texts:
Has there been any theoretical or observational study of the possibility of detecting "local dark matter", either within the solar system or near the Sun in interstellar space?

If so, what methods could possibly be used to detect it?


texts:
Angular momentum, and in particular the conservation of angular momentum, is of course very relevant in many processes in astrophysics, such as e.g. star- and planet formation, and disk formation in galaxies.

The concept of objects spinning up upon collapse is straightforward enough to understand, but this begs the question: where does the primordial angular momentum come from?

Tidal interactions between haloes, I suppose?


texts:
According to the theory of relativity (If I understand correctly), nothing is supposed to be able to travel faster then the speed of light. I believe, to my limited knowledge and grasp of the concept, that quantum theorie speaks against, i.e. extend, the theory of relativity, hence my question:

Suppose we find (or imagine) a place in the universe where there is no obstruction for this thought experiment to take place. We take a spherical object and attach a spire at both sides so you get an object shaped like this: -O-

We start turning the object so the ends of the spire travel in an orbital mather according to the spherical object. What if the sphere starts to orbit near the spead of light, wouldn't the ends of the spires travel faster than the speed of light?

Sorry if this is the wrong place for this question.